You can also get them at Flametree BBQ at AK. They are not a snack credit, you have to pay for them OOP. Can't say they are the best, but they are a CS option.
At the Boardwalk Inn/Boardwalk Villas you can get Onion Rings at Boardwalk To Go which is a food cart. Now they don't accept the Disney Dining Plan, however their cost is only $3.50 which sounds very reasonable if you were to pay out of pocket.
Not sure about the Dining plan option, but Olivia's at OKW and the Turf Club at the Saratoga Springs Resort have the best onion rings and dipping sauces in my opinion